The correct answer is B. With AWS Lambda, you can create test events directly within the AWS Lambda console's 'Test' tab. The shareable event feature allows you to make these test events accessible and editable by IAM users within the same account who have the appropriate permissions. This method leverages built-in AWS Lambda functionality to facilitate easy sharing and modification of test events among all developers in the IAM group. Options A and C require external services and additional configuration, while option D does not specify making events editable by all IAM group members.